Interpretive Summary: We studied the time of transition to reproductive development and the development of floral primordia in three blackberry varieties (Boysen', 'Cherokee', 'and 'Marion') growing in Arkansas and Oregon. Also, the pattern of flower bud development within the long canes was examined. In Oregon bud development in these varieties began in September or October and dcontinued uninterrupted into spring, and was accompanied by increasing complexity in the floral organs. In Oregon daily mean temperatures remained above 38 degrees F throughout the sampling period. In Arkansas bud developed ceased in January and early February when mean daily temperatures fell below 36 degrees F. In all three cultivars throughout the sampling period, buds from the middle section, and sometimes buds from the bottom section, tended to be more advanced in their development than those from the top section of canes. This study has contributed to a better runderstanding of the effect of temperature on flower bud development. Also, for the first time the pattern of bud development along the canes was identified in blackberry varieties of commerical importance. Better under- standing of physiological processes to overcome uneven and poor bud-break will lead to more efficient and economical blackberry production in areas with mild winter conditions.

Technical Abstract:
Flower bud development was studied in 'Boysen', 'Cherokee', and 'Marion' blackberries. In buds of 'Boysen' and 'Marion', both trailing types from Oregon, the transition to reproductive bud occurred in October and sepal primordia were observed by November. Progression of floral bud development continued into January, but at slower rate. Buds on the main canes (> 3-m long) remained at more advanced stages of flower bud differentiation than the buds on the basal lateral canes. In both cultivars, and throughout the sampling period, buds from the middle one-third section, and sometime buds from the bottom one-third section, tended to be more advanced in their development than the buds in the top one-third section of canes. In buds of 'Cherokee' (erect type), the transition to reproductive bud development occurred in September in Arkansas and Oregon. There was clear evidence that the buds in the axils of two or three most basal nodes (proximal to the main cane) and a few buds at most distal nodes lagged behind those bud at middle nodes ( 5 to 13) on ~ 1-m long lateral canes. Along the mid- section of lateral canes, the buds developed rather uniformly during early stages bud development. Buds on lateral canes located proximal (< 1 m) to the ground lagged behind in development compared to those buds from lateral canes sampled from more distal positions on the main cane. The results suggest that the shortening day lengths of late summer trigger flower bud development in blackberry.